The use of cooling tubes in heat exchangers having rectangular or oblong cross-sections such as taught by Wallis in U.S. Pat. Nos. 4,595,135 and 4,971,240 are well known in the art. More recently, Wallis in U.S. Pat. No. 5,271,151 discloses the formation of a plurality of parallel longitudinal flow paths within an oblong cooling tube. Alternatively, Dudley in U.S. Pat. Nos. 5,372,188 and Guntly in 4,998,580 disclose the use of inserts convoluted in a direction parallel to the length of the cooling tube to form the plurality of parallel longitudinal fluid flow paths called turbulators within the cooling tubes, while Study in U.S. Pat. No. 5,456,006 describes a method for inserting a turbulator into the cooling tube during the formation of the cooling tube. In the above cooling tubes the turbulators form small longitudinal flow paths extending parallel to the length of the cooling tube. This configuration is well adapted to charge air coolers.
In an alternate configuration of the cooling tube, Bae in U.S. Pat. No. 5,771,964 teaches a cooling tube in which small fluid flow channels are provided transverse to the length of the cooling tube. This type of geometry produces a more efficient heat exchanger.
Against this background, the need has arisen for cost-effective manufacture of extruded tubes used in heat exchanging, i.e., condensing, applications. One requirement for instance, of refrigeration condensers is to supply the largest surface area to the compressed gases and be able to contain high internal (and often varying) pressures without distortion or leak.